Bias current generators are found in most of today's silicon based integrated circuits. They are designed to provide a bias current to different circuit blocks and find particular use in analog sub-circuits. A bias current is typically generated by reflecting a voltage across a resistor. A very popular bias current generator is based on the base-emitter voltage difference of two bipolar transistors operating at different collector current densities. This voltage is, by its nature, proportional to absolute temperature, denoted as PTAT. There are also bias current generators based on base-emitter voltage or gate to source voltage of MOS transistors. As the base-emitter voltage of a bipolar transistor is complementary to absolute temperature, CTAT, the generated current has a similar temperature dependency and is denoted CTAT.
While these generators find many uses, there are many constraints on modern bias current generators such as: minimum supply voltage, low power, low silicon area for low cost, precision, the capacity to trim the circuits to optimise performance, noise, etc. When a very low power bias current is required the resistor used to convert the voltage in a corresponding current dominates the die area and the associated cost.